We are examining the substrate specificity of bacterial xanthine dehydrogenases and related purine oxidizing enzymes. A survey for the distribution of these enzymes with a large variety of bacteria has revealed that there are at least two patterns of substrate specificity for these enzymes distributed along taxonomic and physiological lines. We have selected strains of Bacillus, Pseudomonas, and anaerobic micrococci which produce high concentrations of the enzyme for study as type examples of these two patterns. The enzymes have been, or are being, purified and the molecular weights, subunit and prosthetic group contents being determined for comparison with each other and the literature. The specificity of the different enzymes will be examined in further detail through a systematic use of substrates and substrate analog inhibitors. Each of these enzymes sources display a characteristic pattern of substrate specificity. Yet, all the enzymes are capable of oxidizing a wide variety of purine and purine analog substrates. The patterns obtained will be interpreted in terms of steric and electronic requirements for oxidation by each of the enzymes studied. A number of kinetic experiments have shown anomalies which suggest a mechanism for the broad specificity pattern of these enzymes. The results suggest that the enzyme preparations contain several forms of the enzyme in equilibrium, each of which displays limited substrate specificty. These investigations will continue. Furthermore, we plan to investigate the possibility of genetic manipulation of the specificty pattern of these enzymes by the selection of mutants resistant to certain of the substrate analogs.